Matt generally means not smooth, which means trying to keep clean is a big pain. Adding wax means shiny, and not matt. Adding a clear top-coat and lightly sanding to make it opaque, would then make it grey, if not whitish. A satin finish clear might help make a flat black cleanable and not too shiny, but not sure. Maybe there's a high-tech paint available that would work for this, but never saw one. Interesting situation, but it begs the question, why in the first place?